Spin-peierls transition in magnetic field

We discuss the characteristic properties of the Spin-Peierls (SP) transition which allow one to distinguish experimentally between such a transition and the ordinary structural instability. It is shown that commensurability effects cause the strong dependence of the transition temperature T c on magnetic field H as well as nonlinear dependence of magnetization on H . They also suppress the phase fluctuations for the SP transition. It is shown also that dimensionless coupling constant of spin-phonon interaction which drives SP transition may be large enough only when the relevant phonon frequencies are extremely small already in a high-temperature phase.